Display screen or portion thereof with a graphical user interface



FIG. 1 is a front view of a display screen or portion thereof with a graphical user interface showing a new design; and,

FIG. 2 is a second image thereof.

The broken lines around the Figures show a display screen or portion thereof and form no part of the claimed design. The broken lines within the Figures show portions of the graphical user interface that form no part of the claimed design.

The appearance of the transitional image sequentially transitions between the images shown in FIGS. 1-2 . The process or period in which one image transitions to another image forms no part of the claimed design. 

CLAIM The ornamental design for a display screen or portion thereof with a graphical user interface, as shown and described. 